Greenhouse system, comprising illumination means for the illumination of plants etc. are of common knowledge.
Recent years have shown increasing practical application of large-scale plant growing systems that make use of artificial light sources, which are less susceptible to the effects of external factors (such as long- and short-term weather conditions and insect damage). The most commonly-used light sources include high-pressure sodium lamps, low-pressure sodium lamps, xenon lamps, metal halide lamps, fluorescent lamps, and microwave lamps. Of these, high-pressure sodium lamps are the most popular because of their relatively high luminous efficiency. Still, an artificial light source such as a high-pressure sodium lamp strikes a poor balance between red (wavelength band: 640 to 690 nm) and blue (wavelength band: 420 to 470 nm), which is important for photosynthesis etc., and therefore the output had to be raised in order to cultivate healthy plants. Furthermore, low-pressure sodium lamps have higher luminous efficiency than high-pressure sodium lamps, but pose problems with optical quality because they output single-wavelength light of the sodium D line, and it is also difficult to raise the output.
Another problem with artificial light sources is that the large amount of radiated heat imposes a heavy load on air-conditioning units, and the plants have to be kept sufficiently far away from the light source in order to keep them from being damaged by this thermal radiation; this tends to result in a bulkier apparatus.
In view of the above problems, artificial light sources that make use of semiconductor based light emitting devices, hereinafter abbreviated as SBLED(s), e.g. light emitting diodes (LEDs), have been proposed in some publications, e.g. in EP1374665. SBLEDs are already available for various wavelengths between about 350-750 nm and in various different embodiments; and the developments are still going on. EP1374667 discloses a mini greenhouse with a LED spotlight. With semiconductor based light emitting devices, there is less load on air-conditioning equipment because elements with a light emitting wavelength band that does not include heat rays can be employed, and the apparatus can also be made more compact, so luminous efficiency is better. Another advantage is that the life of a light emitting diode is several times longer than that of a high-pressure sodium lamp. Also, the shape of the illuminator can flexibly and easily be modified in accordance with the plants being illuminated, and the light density can easily be controlled by arranging numerous semiconductor based light emitting devices in a linear or planar array.
Another advantage of using semiconductor based light emitting devices is that irradiation with pulsed light is possible. When light is pulsed (intermittent light emission), the amount of photosynthesis per unit of light would be increased over that achieved with continuous light, without optical saturation which may occur under intense light. It is stated that the utilization efficiency of light is better using short-period pulsed light than using continuous light.
EP1374667 also describes the use of sensors like an ambient humidity sensor, a temperature sensor and a light sensor in the mini-greenhouse. The devices for heating, ventilation nebulization and irrigation and the lamp can thus be feedback controlled to maintain the corresponding parameters. The temperature sensor commands a heater or a ventilator, the air humidity sensor activates the nebulizer and the soil humidity sensor activates irrigation.
WO 2007/105946 describes a light sensor for use in a greenhouse. The sensor has the structure of an artificial plant, with sensing devices mounted on its leaves. This enables measurements on light that has been filtered by the leaves of the plants. The document describes that the light can be analysed over the entire light spectrum or that parts of the spectrum that are important for the growth of the crop can be analyzed. This gives a three dimensional picture of the incident light over the height of the plants. The document describes that the sensor can be used to influence artificial lighting or to study the development of a crop. Lighting for the plants can be controlled accurately depending on the stage of growth of the plants and the desired yields.
Although WO 2007/105946 mentions that in the end less energy will be required, it does not discuss any specific energy saving measures. Nor does it describe measurement of spectral distribution or its use outside obtaining a three dimensional picture of incident light over the crop height.